Practical aspects of Kelvin probe force microscopy

Abstract

We discuss practical aspects of Kelvin probe force microscopy (KFM) which are important to obtain stable images of the electric surface potential distribution at high spatial resolution (<100 nm) and high potential sensitivity (<1 mV) on conducting and nonconducting samples. We compare metal-coated and semiconducting tips with respect to their suitability for KFM. Components of the metal coating can become detached during scanning, introducing sudden offset jumps in the potential maps (typically up to 350 mV between adjacent scan lines). However, n-doped silicon tips show no substantial tip alterations and, therefore, provide a stable reference during the experiment (offset jumps typically up to 40 mV between adjacent scan lines). These semiconducting tips must be electrically connected via contact pads. We use InGa and colloidal silver pads which are easily applied to the substrate supporting the cantilever and have a low enough differential contact resistance (350 Ω and 2.2 kΩ, respectively). Furthermore, we introduce a simple procedure to fine tune the feedback which detects the electric surface potential and show how the basic KFM setup has to be modified to gain access to the necessary control signals.